The use of emulsions is widespread in cosmetics. An emulsion is understood to be a finely distributed mixture of two normally immiscible liquids without visible separation.
A finely distributed mixture of two liquids, such as, for example, oil and water, is present in an emulsion. In this case, one of the liquids (phase) forms small drops, which are present distributed in the other liquid (phase). The phase forming the droplets is called the internal phase or also the dispersed phase. The phase in which the droplets float is called the external phase or also the continuous phase.
Emulsions belong to dispersed systems and differ from mixtures of miscible liquids. Emulsions are usually cloudy, milky liquids. In the case of emulsions comprising an aqueous phase and an oil phase, oil-in-water emulsions (O/W emulsions) and water-in-oil emulsions (W/O emulsions) are differentiated.
Emulsions are thermodynamically unstable. The dispersed (internal) phase strives to combine by coalescence to form larger areas and in so doing, the interfacial energy between the two phases is reduced. Emulsions used in cosmetics, however, should be preserved usually for a specific time period (between a few hours and a few years) and under certain conditions (e.g., within certain temperature and pH value ranges).
An emulsifier, i.e., a surfactant or a surface-active compound, which facilitates the formation of droplets and counteracts separation (phase separation), is therefore normally used to stabilize the emulsion. Emulsifiers reduce the interfacial tension between the phases by forming interfacial films at the phase boundary between oil and water, as a result of which the irreversible flowing together of the droplets (coalescence) is counteracted. Cloudy emulsions with a droplet size in the micrometer range normally form.
An emulsion can also be stabilized by the addition of specific solids. Solid-stabilized emulsions are often called Pickering emulsions after their discoverer. In 1907, S.U. Pickering demonstrated that small particles that are wetted better by water than by oil can stabilize O/W emulsions. It is important for the sufficient stabilization that a mechanically stable solid film can form around the dispersed phase.
A cosmetic Pickering emulsion can be, for example, an emulsion stabilized by colloidal silica particles. These silica particles arrange themselves at the interface between the two phases (hydrophilic and lipophilic phase) and prevent the droplets of the dispersed internal phase from coalescing (joining of the droplets).
To stabilize the emulsion, Pickering emulsions or solid-stabilized emulsions include particulate solids; said solids can be used either instead of surfactants or also in addition to surfactants. An essential advantage of solid-stabilized emulsions is that the surfactant concentration in the emulsion can be greatly reduced. After reduction of the surfactant concentration as well, solid-stabilized emulsions or Pickering emulsions are generally notable for a very good long-term stability.
A further advantage of Pickering emulsions is their high stability to changes in the chemical milieu such as, for example, a change in the pH values or salt concentrations.
The principle of stabilizing cosmetic emulsions by using particulate solids is already known from the prior art. For example, Pickering emulsions are described in EP 0987002, which are characterized by an oil phase, an aqueous phase, and at least one type of microfine particles with an average particle size of less than 200 nm. The emulsions in EP 0987002 are to be suitable for use as a cleansing emulsion, as a facial or body care preparation, as a sunscreen, or as a deodorant, and in particular have an improved skin tolerance.
Finely divided W/O emulsions with oil droplets in the micrometer range, which are free of surfactants and are stabilized by solids alone, are described in EP 0686391. For stabilization in this case, spherical polyalkylsilsesquioxane particles are employed which have a diameter of 100 nm up to 20 μm. Finely divided O/W emulsions with oil droplets in the micrometer range are described in EP 870495; here, apart from finely divided solids with a diameter of up to 200 nm, surfactants are also used as emulsifiers.
Surfactant-free droplet O/W macroemulsions with an oil droplet particle diameter in the range of 0.1 millimeters to several centimeters, in which finely divided solid particles are also used as an emulsifier, are disclosed in U.S. Pat. No. 3,920,883 and U.S. Pat. No. 4,767,741.
All aforementioned documents describe emulsions that are packaged as a one-component product; i.e., the particular emulsion, whether used as a skin cream, sunscreen, cleansing agent, or as a deodorant as well, is used immediately after removal from the container in which it is provided.
The use of solid-stabilized emulsions is not known so far in cosmetic products the use of which requires prior mixing of two or more separately packaged preparations.
Hair dyes, in particular oxidative hair dyes, are a known example of cosmetic products in which the user must first prepare a ready-to-use mixture shortly before use by mixing various preparations.
Oxidative hair dyes customarily include oxidation dye precursors, so-called developer components and coupler components. The developer components form the actual dyes under the influence of oxidizing agents or atmospheric oxygen with one another or during coupling with one or more coupler components.
If the oxidative dyeing agent includes both oxidation dye precursors and oxidizing agents (such as, for example, hydrogen peroxide), thus both substance classes are packaged expediently separately from one another, so as to prevent a premature, undesirable reaction with one another, and are brought into contact with one another only immediately before use.
In order to prepare a ready-to-use oxidative dyeing agent, the user must therefore mix the first preparation, which includes color-imparting substances (i.e., the oxidation dye precursors and/or the direct dyes) with the second preparation which includes the oxidizing agent.
Generally, both dyeing agent (A) (which includes the oxidation dye precursors and/or the direct dyes) and oxidizing agent preparation (B) are emulsions. In this case, the viscosity of each emulsion must be selected so that both emulsions are thin enough, on the one hand, to assure a complete and homogeneous mixing but, on the other hand, they are adjusted to be sufficiently thick to prevent the dripping of the finished application mixture.
Both dyeing agent (A) and oxidizing agent preparation (B) are therefore optimally adjusted to a precisely defined viscosity range. The mixing of two relatively viscous preparations often poses problems for the user, because mixing of the two preparations becomes the more difficult and more time-consuming, the thicker or more viscous the two preparations are. In particular, if two preparations are to be shaken together, it often occurs that the user ends the mixing or shaking before the application mixture is completely homogeneous. If this nonhomogeneous application mixture is then applied to hair, the outcome is a “variegated” coloring result. When hair is dyed in dark shades, i.e., if especially high amounts of dyes are present in the application mixture, in the case of incomplete mixing an overall too weak color intensity can also result.
Accordingly, it is desirable to provide new dyeing agents for dyeing keratinic fibers that are present in emulsion form, are stable, and have an especially good tolerance to high dye contents. These dyeing agents should be capable of being mixed especially readily and rapidly with a second oxidizing agent preparation, so that after mixing a homogeneous application mixture in the optimally adjusted viscosity range forms. Moreover, the viscosities of the dyeing agent and the finished application mixture should be within the desired specification range after a rather long storage time as well; i.e., the viscosities of dyeing agents and the application mixture should not change or shift in an unforeseeable manner also after weeks of storage of the dyeing agent. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with this background of the invention.